Some tuning technologies described below are mainly used in external cavity tunable lasers. First, tuning is carried out by using a precision stepping motor to drive a grating to rotate, and this technology has the following shortcomings: 1), there are high requirements on precision and repeatability of the stepping motor in achieving optical frequency precision tuning, thus the cost is high; 2), the purpose of miniaturization is hardly achieved due to the stepping motor used; and 3), the working stability is poor under a harsh working environment, in particular, prone to various mechanical vibrations. Because of these problems, the tunable laser using this technology is only suitable for use under a laboratory working environment. Second, tuning is carried out based upon the temperature-based shift of the optical frequency of grating or other optical filtering devices in laser resonant cavity, such as an etalon. This tuning technology has high tuning precision and relatively narrow spectrum bandwidth of output light, but low tuning speed. This shortcoming becomes noticeable especially in the case that wide spectral range tuning is needed. For example: if the temperature shift coefficient of an optical filtering device is 0.2 nanometers/degree, the desired spectrum tuning range and temperature adjustment range are 80 nanometers and 400 degrees respectively, which is impracticable in practical application. Third, tuning is carried out by Micro Electronic Mechanical System (MEMS). This technology has a main shortcoming that the working stability is very poor under a harsh working environment, in particular, prone to various mechanical vibrations. Fourth, tuning is carried out by a tunable acousto-optic filter. This technology has the advantages of high tuning speed, no mechanical movement component and small size, but low tuning precision and relatively wide filtering bandwidth. Therefore, the tunable laser using this technology is only suitable for applications in which the requirement of tuning precision and the output bandwidth are not high. Finally, the tunable lasers using a single laser gain medium can hardly cove both the C and L bands.
To sum up, the existing technologies cannot satisfy a variety of applications of the tunable lasers in which miniaturization, fast tuning within a wide spectrum range, narrowband laser output and long-term stable working under a harsh environment are required, especially for applications in fiber optical communication filed.